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Precision Steering at 18,000 M.P.H.
September 1957 Radio & TV News Article

September 1957 Radio & TV News

September 1957 Radio & TV News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919 - 1959. All copyrights hereby acknowledged.

These days, I'm always a bit hesitant to publish or do Internet research on items mentioning chemical compounds any more toxic than rubbing alcohol. This article reports on early plans of the Vanguard satellite launch platform for America's first orbiting satellites. Although the main focus is on the electronic steering and stabilization systems, it mentions the fuel composition of nitric acid and unsymmetrical dimethylhydrazine. That's probably enough to cause a federal agent to come knocking on my door... or at least put me on some sort of surveillance list. Less exotic fuels like LOX (not to be confused with the fish used on bagels), kerosene, and liquid hydrogen, which powered most of the man-carrying booster stages, would not likely raise a flag.

Precision Steering at 18,000 M.P.H.

Precision Steering at 18,000 M.P.H., September 1957 Radio & TV News - RF CafeBy Otto Berger

Servo Section, Project Vanguard

The Glenn L. Martin Company

The vehicle that will launch the space satellite is right out of "science fiction", involving as it does the ability to "think" and cope with the problems of "spacemanship".

At this moment small groups of rocketry specialists in engineering centers across the United States are pouring the full measure of their sweat and ingenuity into a project called "Vanguard." Their assignment is to design and test the rocket vehicle that will launch man's first satellite.

The date of the first launching is fast approaching. President Eisenhower has announced that the United States will attempt to launch several small, unmanned earth-circling satellites during the International Geophysical Year, July 1, 1957 to December 31, 1958.

The project is proceeding under management of the Naval Research Laboratory, supported by agencies of the Army and Air Force. The Martin Company of Baltimore is prime contractor, charged with responsibility for the design and manufacture of the vehicle that will place the satellite in its orbit. The National Academy of Sciences, through the Naval Research Laboratory, will provide the satellite itself and its instrumentation.

"Project Vanguard" has been hailed by scientists as a mission of great peacetime promise. Artificial satellites are destined to be man's first observation posts operating for sustained periods of time beyond the atmosphere. From them will flow an abundance of new knowledge relating to the earth and the universe.

Yet the day of the satellite would still be a long way off if it were not for the great strides made recently in rocket propulsion, structural design, and electronics. A satellite launching system draws upon these technologies to the limits of their development. "Project Vanguard" then, a forerunner of future progress, is no less a sign of present achievement.

The "Vanguard" vehicle is a three-stage rocket powerful enough to vault through the earth's atmosphere to orbiting altitude of 300 miles. If it did no more, the satellite would immediately fall back to earth. It must, therefore, be able to accelerate to the amazing velocity of 18,000 miles-per-hour - the rate that offsets the centripetal pull of gravity at that altitude, and thereby makes orbital travel possible. The 11-ton vehicle must have a means, moreover, of controlling this great lifting strength and velocity so that the satellite will follow a path that roughly parallels the earth's contour.

Three big demands are thus laid down for the satellite vehicle. It must lift the satellite to a height of 300 miles; accelerate it to 18,000 miles-per-hour; and then - at that altitude and velocity - it must set the satellite free on a path that approximates a tangent to the earth's surface. That these capabilities may be built into a single vehicle of manageable size and cost is a tribute to our state of advancement in rocketry, electronics, and allied fields.

Designed and built by Aerojet-General Corporation - RF Cafe

The second stage of the vehicle, shown at left, contains a liquid rocket engine, designed and built by Aerojet-General Corporation. A gimbal mounting system and hydraulic actuation units similar to those employed in the first stage are used for control of the thrust vector during the second stage burning cycle.

The Three Stages

The composite vehicle, resembling a gigantic rifle shell, is about 72 feet long and 45 inches at its greatest diameter. The first two stages are powered by liquid propellants and guided by an inertial reference system. The third stage, which carries the spherical satellite, is powered by solid propellant and is maintained in fixed orientation while it is firing.

The first stage is a liquid propellant rocket similar to the "Viking" built by Martin for the Navy, but with substantial improvements. Serving essentially as a guided booster, it develops most of the energy to raise the remaining stages to orbital height and about 15% of the required orbital velocity. The engine, built by General Electric Company, delivers a thrust of approximately 27,000 pounds at sea level. The major propellants, liquid oxygen and kerosene, are contained in tanks that are integral with the airframe skin. The rocket motor is fed fuel by turbine-driven pumps. The pressurizing gas is helium. Control of the vehicle's orientation and flight path is attained by movements of the engine which is mounted on a gimbal. In response to autopilot commands, the engine is tilted by electro-hydraulic actuators to alter the direction of thrust and thus control deviations in pitch and yaw. Roll control is provided by small auxiliary jet reactors.

The second stage of "Vanguard" carries the entire guidance and control system. In addition it supplies the remaining energy needed to reach orbital height, and about 30% of the orbital velocity. It is a liquid propellant rocket that is spliced to the forward end of the first stage. The propellants, nitric acid and unsymmetrical dimethylhydrazine, are fed directly to the motor from high pressure tanks integral with the airframe skin. Again the pressurizing gas is helium. The motor is gimbal-mounted, as in the first stage, and positioned in pitch and yaw by electro-hydraulic impulses. An array of jet reactors provides complete control of orientation during second-stage coasting flight. Forward of these various mechanisms, the second stage houses within its nose - which is the nose of the entire vehicle - the third stage and the satellite.

The plastic nose cone protects the delicate satellite sphere from the aero-dynamic heating it would encounter if exposed during the first part of the ascent through the atmosphere. The cone is jettisoned early in the second stage burning phase, after which the atmosphere is too thin to be detrimental to the satellite.

Artist's concept of the satellite preliminary trajectory - RF Cafe

Artist's concept of the satellite preliminary trajectory.

The third stage is a solid-propellant, unguided rocket. By approximately doubling the speed attained by the end of second-stage coasting flight, it imparts the 18,000 mile-per-hour velocity required for the satellite to begin its free-flight orbit around the earth. In the absence of guidance-jettisoned at the time of second stage separation - this third stage maintains stability by being spun about its longitudinal axis in the manner of a rifled shell. While still attached to the second stage, it is mounted on a turntable, or spinning mechanism. Near the end of second-stage coasting flight, the turntable is set in motion by small solid propellant rockets. When the third stage is. spinning, retro-rockets fire-retarding the flight of the second stage shell. The momentum of the third-stage-satellite combination, however, remains unchecked. Thus freed, the final rocket begins its powered flight.

The satellite payload, a 20-inch sphere, is attached to the forward end of the third stage, and may be separated when orbital velocity has been attained. As the third stage will reach orbital velocity, when separated from the payload, it also will become a satellite.

Orbital Characteristics

Even at altitudes of 300 miles and above there is a minute drag. Over a period of time this drag will retard the satellite's velocity and thus lower its altitude, so that it will describe a de­celerating, descending spiral. When it descends to atmosphere of sufficient density, the satellite will burn and dis­integrate. Based on present estimates of densities, scientists at the Naval Research Laboratory calculate that the satellite could exist in a circular orbit of 300 miles height about one year. If the height varies from 200 to 1500 miles at the lowest and highest points (perigee and apogee), the lifetime would be only 15 days. A 100-mile perigee would end the satellite's career within an hour.

Liquid oxygen and kerosene as fuels - RF Cafe

The first stage (portion of complete vehicle shown at the right) is powered by a liquid rocket engine, employs liquid oxygen and kerosene as fuels. The thrust cylinder extends aft of the rocket structure. This cylinder is moved by hydraulic actuators in a gimbal system so that flight path control is possible.

The preferred orbit - a nominal circle 300 miles above the surface of the earth - could be attained only if the angle and velocity of firing were controlled perfectly. Inevitable control errors, however, will result in an elliptical orbit.

It is intended that the initial orbit will lie between 200 and 1500 miles altitude. A greater apogee would hinder optical tracking while a perigee below 200 miles would seriously reduce the life span of the man-made moon.

Control System

Correct angle of injection depends on correct functioning of the control system which steers the "Vanguard" vehicle over the predetermined trajectory. It employs a magnetic amplifier autopilot working in conjunction with an inertial reference guidance system. The course is set into the system before launch and played back via a master sequence controller which initiates each phase of flight at precisely the right moment. It is thus unlike other guidance systems that employ radar to track the rocket, issuing steering commands from the ground with the help of a computer.

All control equipment is located in the electronic section of the second stage. Heart of the guidance system is a trio of single-axis, rate-integrating gyros. One is aligned with the "yaw" axis, another with the "pitch" axis, and the third with the "roll" axis. Once set and stabilized in a particular plane, the gyros remain fixed in that plane despite contrary movements of the vehicle. Roll and yaw orientation are fixed, while the pitch reference is pre-programmed to establish the curving trajectory planned for the rocket.

Let's say the heading of the vehicle changes from the desired direction because of a gust of wind, sloshing of fuel in the tanks, or irregularity of the rocket engine. The deviation is sensed by the yaw gyro, which remains set on the correct course. The gyro sends out proportionate electrical signals to the autopilot, which, operating through electro-hydraulic actuators, causes the rocket controls to bring the vehicle back on course. Deviations in roll and pitch are corrected in similar fashion.

Equipment that will be used to launch the satellite-carrying Vanguard three-stage vehicle - RF CafeThis is the equipment that will be used to launch the satellite-carrying Vanguard three-stage vehicle. The structure in the background is a gantry crane used to erect and assemble the vehicle and to provide work platforms from which the field crew can test the rocket prior to launch. At the lower right is the concrete blockhouse from which the rocket operation is monitored prior to and during flight. All final tests are remotely performed by personnel locked in the blockhouse and when the rocket is ready for flight it is fired from this remote location. As seen in this sketch, the Vanguard launch stand does not include a pit to conduct exhaust gases away from the rocket but rather the vehicle itself will be elevated and steel exhaust duct provided. The rocket will be finless and stabilization will be achieved by use of a gimbaled engine. Omission of fins saves weight.

The phase lead required to stabilize the rocket is produced by operational networks that introduce a phase lag in the feedback circuit of the amplifiers.

Using the conventional equations for a feedback amplifier:

     - RF Cafe

where: Kƒ=gain with feedback

    K = forward gain

     - RF Cafe

where: Kb is a function of the number of turns on the feedback winding and the values of the resistances.

On substitution we obtain: 1 + (RCS/2)

   - RF Cafe

When compared with a conventional lead circuit, this equation shows that the time constant, T, is given by (RCS/2) and the attenuation by 1 + KKb.

Velocity Measurement

The all-important velocity of the "Vanguard" vehicle is measured by an integrating accelerometer installed in the second stage electronics section. The instrument senses the acceleration applied to the vehicle during flight, sums it up, and thus yields the velocity. The velocity measurement made at the end of the burning of the second stage is supplied to the unit's analogue computer, which determines how long the vehicle will coast before the third stage, carrying the satellite, is fired.

The basic component is a floated gyro. An acceleration on its sensitive axis generates a signal which, when amplified, drives a turntable which rotates the gyro about its input axis. The resulting torque is equal and opposite· to the acceleration torque. Since the turntable turns at a rate proportional to acceleration, its position is proportional to the integral of acceleration, or velocity.

The relation between torque and input, angular velocity depends on the angular momentum which, in turn, depends on the power frequency. There is no means of compensating for changes in frequency. Consequently, since the frequency of the 400 cps supply is not controlled accurately enough, it is necessary to generate an accurate 400 cps. This is done by means of a tuning fork whose output is amplified in a transistor amplifier to drive the gyro.





Posted  August 26, 2014

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